L 26

Respiratory System

• This presentation covers the respiratory system, focusing on the pleura, lung compliance, and pulmonary surfactant.

Learning Outcomes

• To understand the pleura and diaphragmatic recesses
• To define lung (pulmonary) compliance and its determinants
• To describe the role of elastic fibers in lung parenchyma compliance
• To elucidate the role of pulmonary surfactant concerning surface tension and its clinical ramifications

Pleura

• The pleura is a closed serous sac.
• It has two layers: visceral and parietal pleurae.
• These layers are continuous around the lung roots and pulmonary ligaments.
• Visceral pleura closely adheres to the lung, while the parietal pleura lines the thoracic cavity.

Pleura - Layers

• Parietal pleura is subdivided into costal, diaphragmatic, cervical and mediastinal pleurae.
• Costal pleura lines the inner surface of the sternum, ribs, costal cartilages, and intercostal spaces.
• Diaphragmatic pleura covers the thoracic surface of the diaphragm.
• Cervical pleura extends from the first rib to the apex of the lung, connecting with the mediastinal pleura.
• Mediastinal pleura forms the lateral boundary of the mediastinum and covers the mediastinal surface of the lungs.

Recesses of Pleura

• Pleural recesses are widenings in the pleural cavity.
• They act as reserve spaces for lung expansion during deep inspiration.
• Costomediastinal recess: Between costal and mediastinal pleurae.
• Costodiaphragmatic recess: Between costal and diaphragmatic pleurae. These recesses are prominent in reference to the cardiac notch of the lung. At mid-clavicular, mid-axillary, and scapular lines respectively, the lower limit of the pleura is along ribs 8, 10 and 12. The lower border of the lung follows ribs 6, 8 and 10.
• These recesses allow for maximum lung expansion during inspiration. If fluid leaks into the pleural cavity, this fluid first collects in the costodiaphragmatic recess.

Determinants of Lung Compliance

• Lung compliance is the measure of lung expansibility.
• Two factors determine lung compliance
  • Elastic Property of the lung.
    • Tissue elastic properties due to elastin and collagen fibers in lung parenchyma and elastic forces from the surface tension of lung alveolar fluid.
  • Elastic property of the thoracic cage is due to the rib's elastic nature, muscle, and tendon

Lung Compliance

• The total compliance of both lungs in a normal adult is 200 ml of air per cm of water transpulmonary pressure.
• Tissue elastic forces account for 1/3 of the total lung elasticity, while fluid surface tension elastic forces contribute 2/3.
• The total amount of work done during breathing is the loop area.

Pressure-Volume Curve

• Lung compliance is a static measurement of lung and chest recoil (or ability to return).
• The curve during inflation differs from the curve during deflation, which is termed hysteresis.
• Lung volume at any given pressure during inhalation is less than the lung volume at the same pressure during exhalation.

Compliance Calculations

• Normal Compliance: 0.1 liter/cmH2O
• Low compliance: 0.05 liter/cmH2O
• High compliance: 0.17 liter/cmH2O

Factors Affecting Lung Compliance

• Decreased compliance: pulmonary congestion, interstitial pulmonary fibrosis, supine position, restrictive lung disease, pneumothorax, hydrothorax, asthma.
• Increased compliance: emphysema, old age.

Surface Tension

• Alveoli are lined with a thin layer of fluid with an interface between air and water.
• Water molecules at the surface of the alveolar fluid have inward-pulling forces (surface tension) causing the alveoli to collapse.
• Surfactant is a key factor preventing alveolar collapse.

Action of Surfactant

• The presence of surfactant reduces surface tension.
• Surfactant content in each alveolus is relatively constant. Lowering of surface tension when alveolus size decreases results in a similar pressure increase.
• The action of surfactant is regulated by the law of Laplace (P = 2T/r).

Functions of Alveolar Surfactant

• Stabilizes alveoli of varying radii.
• Prevents collapse of alveoli during expiration.
• Prevents pulmonary edema.
• Increases lung compliance by reducing the centripetal force of surface tension in alveoli and increasing the lungs' stretchability.
• Regulates lung inflammation and facilitates phagocytosis.

Surfactant

• A lipid substance that reduces surface tension.
• Primarily dipamitoylphosphatidylcholine (DPPC)
• Mixture of other lipids and proteins
• Prevents alveoli from collapsing during expiration and prevents pulmonary edema

Formation of Surfactant

• Surfactant components are synthesized from precursors in the endoplasmic reticulum and transported via Golgi apparatus.
• Transported through multi-vesicular bodies and packaged in lamellar bodies.
• Release via secretion (exocytosis).
• Components organize as a lattice (tubular myelin) within the alveoli.
• This is believed to generate phospholipids necessary to create a monolayer at the air-liquid interface, lowering surface tension.

Metabolism of Surfactant

• Phospholipids and proteins are subsequently absorbed by Type II cells, stored in lamellar bodies, then recycled and returned to the alveolar lumen.
• Alveolar macrophages may take up some surfactant in the liquid layer as well.

Respiratory Distress Syndrome (IRDS)

• A syndrome in newborns characterized by poor lung function due to the lack of sufficient surfactant.
• Interventions to minimize risk:
  • use of antenatal steroids
  • appropriate resuscitation and use of CPAP
  • Early administration of surfactant

This quiz covers essential aspects of the respiratory system, including the pleura, lung compliance, and the role of pulmonary surfactant. Test your understanding of the pleura's structure and function, lung compliance determinants, and clinical implications of surfactant in respiratory health.